The present invention relates to the field of photonic band gap fibers and, more particularly, to the use of temperature tuning of a filled lattice structure to modify the dispersion characteristics of the photonic band gap fiber.
Photonic band gap (PBG) structures consist of a crystal-like periodic arrangement of materials having different refractive indices. Bragg reflections off structures having periodicity extending in more than one dimension can result in the complete reflection of light at certain frequencies, independent of the angle of incidence. The structure is said to possess a band gap at these frequencies. If there is a defect in the periodicity of the structure, then light frequencies lying in the band gap are confined to the defect by Bragg reflection from the surrounding crystal structure, thus making the defect a light guide.
In a PBG fiber, confinement of light to the core region will therefore occur through reflections off of the two-dimensional microstructure surrounding the core. This unique confinement mechanism, as opposed to total internal reflection (TIR) guidance in conventional optical fibers, gives rise to interesting dispersion properties in PBG fibers that are strongly influenced by the crystal structure. Heretofore, research on PBG fibers has focused on the presence or absence of the band gaps, and little work has been done on studying the properties of the guided light. One critical characteristic that has not been properly analyzed, either theoretically or experimentally; is dispersion (the broadening of signal pulses during propagation along a PBG fiber). In particular, there is little known of how dispersion varies with wavelength across the band gap and, as a result, fibers with dispersion appropriate for transmitting communications signals have not been specified. In particular, transmission applications require a low and flat dispersion, whereas dispersion compensation or other device applications may require high dispersions.
Thus, a need remains in the prior art for an arrangement to control and/or modify the tunable dispersion range in a photonic band gap optical fiber.
The need remaining in the prior art is addressed by the present invention, which relates to the field of photonic band gap (PBG) fibers and, more particularly, to the use of temperature tuning of a filled lattice structure to modify the dispersion characteristics of the PBG fiber.
In accordance with the present invention, a PBG fiber is formed to comprise a solid silica core region surrounded by a lattice of air holes formed in the silica material. The holes are filled with a material having a relatively high index of refraction (higher than the silica core), where the precise value of the index of refraction is a function of temperature. The higher index of refraction in the lattice surrounding the low-index core ensures that the band gap effect is the only possible guidance mechanism for optical signal propagation (as opposed to TIR for a conventional optical fiber). It has been found that varying the temperature of the filled PBG fiber leads to changes in the photonic band gap spectra, such as changes in both the spacing and width of the band gaps. Therefore, the ability to xe2x80x9ctunexe2x80x9d the photonic band gap by adjusting the temperature results in the ability to form tunable filters and dispersion compensators in PBG fiber.
In one particular embodiment of the present invention, an oil exhibiting an index of refraction (at room temperature) of n=1.80 at a wavelength of 589 nm is used to fill the air holes, the oil exhibiting a change in refractive index as a function of temperature (dn/dT) of approximately xe2x88x926.8*10xe2x88x924/xc2x0 C., as compared with the temperature-dependent change in the index of refraction for silica glass (dn/dT=1.19*10xe2x88x925/xc2x0 C.).
Other and further embodiments of the present invention will become apparent during the course of the present invention, and by reference to the accompanying drawings.